When ore is dry-ground in a closed cycle to pelletizable particles it is necessary for the production of pellets of uniform quality to obtain a finished product which is as uniform as possible and at a rate which is as constant as possible. For a good finished product, the material discharged from the mill must be classified and the coarse fraction obtained by the classification must be recycled to the mill. Also, the rate at which material is fed to the mill must be maintained as constant as possible.
To feed material to the mill at a constant rate, it is known to maintain a constant total rate of ore feed and recycled coarse fraction and to change the ore-feed rate in compensation of a change of the coarse fraction recycle rate (Printed German Application 1,208,160; Printed German Application 1,002,596; Printed German Application 1,182,032).
Even if the nature of the ore and the operating conditions remain the same, there will be periodic fluctuations of the rate at which the coarse fraction becomes available. In the known process these fluctuations are compensated immediately by a corresponding change of the ore feed rate. This automatic control results in variations in the overall system, and these variations can often build up so strongly that the operation is temporarily terminated or restricted. The quality of the finished product also varies very strongly.